Stall occurs in gas turbine engines when the compressor pressure ratio initially exceeds a critical value at a given speed, resulting in reduced flow capacity and efficiency. This causes a number of compressor blades to "stall" with a resulting momentary compressor airflow reversal.
A stall/surge event may only take 50 milliseconds from beginning to end, although a series of these events may occur in rapid succession. If the stall is undetected and allowed to continue, the combustor temperatures and the vibratory stresses induced in the compressor may become sufficiently high to cause engine damage.
An engine experiencing a recoverable stall will return to normal operation on its own, although the pilot may experience a noticeable loss of power. In contrast, a nonrecoverable stall cannot automatically correct itself and requires the pilot to turn off and restart the engine.
A stall may be alleviated by reducing the fuel to the burners or by bleeding a portion of the compressor airflow. Either can be performed automatically by the fuel control. Alternatively, manual corrective action can be taken, e.g. the pilot cutting back on the throttle. In each case, a stall signal must be provided to the control.
In a craft, e.g. a helicopter, not equipped with a stall detection system, the pilot must monitor various parameters and decide on the incipiency of stall. However, this method is error-prone due to the rapidity with which the stall condition manifests itself. Thus, it is desired to have an automatic stall detection system on board to accurately detect the stall incipiency.
Prior art stall detection systems typically sense a number of engine parameters and make a stall determination therefrom. However, these systems have varying degrees of stall predictability. For example, it is known to determine a stall from certain ranges of one or two parameters. However, this may give false stall indications since the parameter ranges may also be indicative of conditions other than stall. Also, a stall detection system using a small number of parameters is less sensitive to incipiency of stall and has less ability to operate under changing flight conditions. Further, since some parameters are worse indicators of stall than others, the use of these parameters increase the time to detect a stall. Thus, it is desired to improve upon the response time of these systems in making a fast and accurate determination of stall incipiency.
Once detected, the stall signal may be incorporated in a stall recovery system that initiates an automatic stall recovery sequence by, e.g. shutting off fuel, starting ignition, and reinitiating fuel flow (e.g. U.S. Pat. No. 4,118,926). However, such response is undesirable due to the loss of thrust.